The goal of this R03 proposal is to identify molecular mechanisms underlying the regulation of inhibitory synaptic transmission by the cell adhesion molecule (CAM) neuroligin 2 (NL2). Although it has been established that neuroligins mediate synaptogenic activity, it is not known if neuroligins also participate in events that functionally regulate synaptic transmission once the synapse is formed. In our preliminary studies, NL2 was sufficient to induce GABAergic synapse formation in a co-culture system between cultured neurons and HEK293 cells expressing NL2 and GABAA receptors (GABAARs). In addition, NL2 over-expression induces a significant increase in frequency and peak amplitude of miniature inhibitory postsynaptic currents (mIPSCs) in cultured cerebellar granule cells (CGCs). Strikingly, the decay of mIPSCs in the NL2 overexpressing CGCs is significantly faster than that seen in control CGCs. We and others have shown that the switch of 1 subunits of GABAA Rs from 12/3 to 11 underlies developmental speeding of the IPSC decay in various CNS regions. These results lead to our main hypothesis that NL2 facilitates GABAergic synapse maturation and accelerates synaptic decay by recruiting 11 subunit containing GABAA receptors at postsynaptic sites, and that this recruitment occurs via the NL2 interacting scaffolding protein S- SCAM. To test this hypothesis, we will pursue three specific aims: (1) To determine whether altered GABAAR subunit composition accounts for the acceleration in decay time of GABAergic synaptic currents induced by NL2;(2) To define the intracellular domains of NL2 that are responsible for recruiting 11 subunit containing GABAARs at postsynaptic sites;(3) To determine the role of the S-SCAM, the main NL2 interacting protein, in recruitment of 11 containing GABAARs at postsynaptic sites by NL2. Synapse development is a crucial process in the generation of neuronal circuits in the brain. Aberrant synapse development and synaptic dysfunction may lead to severe neurodevelopmental disorders, such as autism, Asperger syndrome and mental retardation. These studies will enable us to understand how neuroligins regulate inhibitory synaptic transmission, and, therefore, serve to further our understanding of the functional role of neuroligins at synapses. Because of their complicated etiologies and various clinical symptoms, treatments of neurodevelopmental disorders are still far from satisfaction. Elucidating functional role of neuroligins at inhibitory synapses will no doubt increase our ability to treat this group of diseases. PUBLIC HEALTH RELEVANCE: Synapse formation and maturation are crucial processes in the generation of neuronal circuits in the brain. Aberrant synapse development and synaptic dysfunction may lead to neurodevelopmental disorders such as autism, Asperger syndrome and mental retardation. These diseases are characterized by impaired social interaction, communication deficits, and repetitive behaviors. Because of their complicated etiologies and various clinical symptoms, techniques of diagnosis and treatment of development disabilities are still far from satisfaction. With the rapid development in neuroscience research, it becomes clear that all neurodevelopmental disorders have corresponding functional changes in one possible site: the synapse and its development. Recent findings have shed some new light on the involvement of members of the neuroligins (NLs) in synapse formation. NLs are postsynaptic cell adhesion molecules (CAMs) expressed on the surface of postsynaptic membrane. NLs bind to their presynaptic partners, neurexins, on presynaptic nerve terminals. The pairing between NLs/neurexins spans the synaptic cleft and forms a physical tether. The function of NLs in synaptogenesis might not be restricted to excitatory synapses as previously anticipated. Down-regulation of NLs results in a loss of both excitatory and inhibitory synapses. Indeed, NL2, one of the family members of NLs, is found enriched at inhibitory synapses. We and others have shown that NL2 promotes the initial steps in GABAergic synapse formation, presumably via trans-synaptic interactions with neurexins expressed on axonal processes in contact with transfected HEK293 cells. However, one important question is still remained: do NLs regulate synaptic transmission once the synapse is formed? Our striking preliminary studies demonstrated that the decay of mIPSCs in the NL2 overexpressing cultured cerebellar granule cells (CGCs) is significantly faster than that seen in control CGCs. These results lead to our main hypothesis that NL2 facilitates GABAergic maturation and accelerates synaptic transmission by recruiting synaptic GABAA receptors with fast decay time component. To test this hypothesis, threes aims will be pursued in this current proposal: (1) To determine whether altered GABAAR subunit composition accounts for the acceleration in decay time of GABAergic synaptic currents induced by NL2;(2) To define the intracellular domains of NL2 that are responsible for recruiting 11 subunit containing GABAARs at postsynaptic sites;(3) To determine the role of the S-SCAM, the main NL2 interacting protein, in recruitment of 11 containing GABAARs at postsynaptic sites by NL2. We believe that the studies we proposed above will enable us to understand how neuroligins regulate inhibitory synaptic transmission, and, therefore, serve to further our understanding of the functional role of neuroligins at synapses. Though it is clear that there is a strong genetic component to neurodevelopmental disorders, their complicated etiologies and various clinical symptoms make the treatments of neurodevelopmental disorders far from satisfaction. Elucidating functional role of neuroligins at inhibitory synapses will no doubt increase our ability to develop more effective diagnosis and treatment methods for this group of diseases.